Specialized drill bits are used to drill well-bores, boreholes, or wells in the earth for a variety of purposes, including water wells; oil and gas wells; injection wells; geothermal wells; monitoring wells, mining; and, other similar operations. These drill bits come in two common types, roller cone drill bits and fixed cutter drill bits.
Wells and other holes in the earth are drilled by attaching or connecting a drill bit to some means of turning the drill bit. In some instances, such as in some mining applications, the drill bit is attached directly to a shaft that is turned by a motor, engine, drive, or other means of providing torque to rotate the drill bit.
In other applications, such as oil and gas drilling, the well may be several thousand feet or more in total depth. In these circumstances, the drill bit is connected to the surface of the earth by what is referred to as a drill string and a motor or drive that rotates the drill bit. The drill string typically comprises several elements that may include a special down-hole motor configured to provide additional or, if a surfaces motor or drive is not provided, the only means of turning the drill bit. Special logging and directional tools to measure various physical characteristics of the geological formation being drilled and to measure the location of the drill bit and drill string may be employed. Additional drill collars, heavy, thick-walled pipe, typically provide weight that is used to push the drill bit into the formation. Finally, drill pipe connects these elements, the drill bit, down-hole motor, logging tools, and drill collars, to the surface where a motor or drive mechanism turns the entire drill string and, consequently, the drill bit, to engage the drill bit with the geological formation to drill the well-bore deeper.
As a well is drilled, fluid, typically a water or oil based fluid referred to as drilling mud is pumped down the drill string through the drill pipe and any other elements present and through the drill bit. Other types of drilling fluids are sometimes used, including air, nitrogen, foams, mists, and other combinations of gases, but for purposes of this application drilling fluid and/or drilling mud refers to any type of drilling fluid, including gases. In other words, drill bits typically have a fluid channel within the drill bit to allow the drilling mud to pass through the bit and out one or more jets, ports, or nozzles. The purpose of the drilling fluid is to cool and lubricate the drill bit, to stabilize the well-bore from collapsing, to prevent fluids present in the geological formation from entering the well-bore, and to carry fragments or cuttings removed by the drill bit up the annulus and out of the well-bore. While the drilling fluid typically is pumped through the inner annulus of the drill string and out of the drill bit, drilling fluid can be reverse-circulated. That is, the drilling fluid can be pumped down the annulus of the well-bore (the space between the exterior of the drill pipe and the wall of the well-bore), across the face of the drill bit, and into the inner fluid channels of the drill bit through and up into the drill string.
Roller cone drill bits were the most common type of bit used historically and typically featured two or more rotating cones with cutting elements, or teeth, on each cone. Roller cone drill bits typically have a relatively short period of use as the cutting elements and support bearings for the roller cones typically wear out and fail after only 50 hours of drilling use.
Because of the relatively short life of roller cone bits, fixed cutter drill bits that employ very durable polycrystalline diamond compact (PDC) cutters, tungsten carbide cutters, natural or synthetic diamond, other hard materials, and combinations thereof, have been developed. These bits are referred to as fixed cutter bits because they employ cutting elements positioned on one or more fixed blades in selected locations or randomly distributed. Unlike roller cone bits that have cutting elements on a cone that rotates, in addition to the rotation imparted by a motor or drive, fixed cutter bits do not rotate independently of the rotation imparted by the motor or drive mechanism. Through varying improvements, the durability of fixed cutter bits has improved sufficiently to make them cost effective in terms of time saved during the drilling process when compared to the higher, up-front cost to manufacture the fixed cutter bits.
Unfortunately, fixed cutter bits have several disadvantages. The first is that fixed cutter bits often have problems with stability while drilling. Specifically, fixed cutter bits often undergo what is referred to as whirl and/or dynamic instability, which often is characterized by shocks, or chaotic movement of the drill bit within the well-bore that takes the form of suddenly stopping, i.e., rotation momentarily ceases at the drill bit or at just a portion of the drill bit but not within the drill string; sudden release of the energy stored within the drill string when the bit begins to rotate again; uncontrolled and rapid movement laterally against the wall of the well-bore; and bouncing, or rapid movement in the longitudinal direction parallel to the long axis of the well-bore. The severity of these movements can exceed 100 times the force of gravity and can damage the drill bit, the drill string, surface equipment, and other items. In addition, the excess energy released in these various shocks is not used to drill the well-bore, resulting in a slower rate of drilling, or rate-of-penetration (ROP), and possibly damaging the cutters and/or the drill bit, leading to increased drilling costs.
Various methods have been attempted to reduce the occurrence of whirl and/or dynamic instability, but none have been wholly satisfactory. Computer modeling to balance the anticipated forces on the drill bit provides some improvement, but cannot account for the variety of factors encountered during the drilling process. Using more, smaller diameter cutting elements and more blades on the bit improves the stability of the bit because more points of contact between the drill bit and the well-bore exist, but such a configuration typically costs more to manufacture and reduces the rate at which the fixed cutter bit drills the well-bore, thereby increasing the total cost. Conversely, using a fixed cutter bit with larger diameter cutting elements and fewer blades and/or fewer number of cutters typically improves the rate-of-penetration and lowers the cost to manufacture the bit, but stability is reduced.
In addition to resisting whirl and/or dynamic instability, the drill bit is part of a dynamic system with both known and unknown inputs. While the inputs into the system at the surface may be known, e.g., type of bit, force or weight applied to the bit at the surface, torque applied at the surface, the actual effect of these surface inputs is typically more variable and less predictable at the drill bit and is only occasionally known through the use of specialized measurement tools located near the drill bit that are capable of transmitting that information to the driller/user at the surface. Such specialized tools are rarely run because of the cost, thus the actual conditions and inputs to which the bit is exposed is typically unknown or known only in partial detail, thus requiring educated guess-work to modify the inputs to improve the operation of the drill bit.
Unfortunately, drill bits typically have a small range of operating conditions in which they operate effectively, such as remaining stable while rotating (which is more than just avoiding whirl) and efficiently drilling subsurface geological formations. Thus, there exists a need for a drill bit that operates efficiently and remains rotational stable over a wide range of conditions.
Further compounding the above problems, drill bits that drill (optimally) a round bore-hole have cutters located at the center of the drilling face, or crown, as will be described below. As these cutters are aligned with or within a close radial distance to the axis of rotation, these cutters have a proportionally low rotational velocity compared to those cutters located at or near the maximum radial distance from the center of the drill bit. This makes the drilling or cutting of the formation near the axis of rotation correspondingly more difficult.
Thus, there exists a need for a cost-effective, stable fixed cutter drill bit that provides improved stability and improved ability to cut or drill a formation near the axis of rotation of the drill bit without sacrificing rate-of-penetration.